Heart rate variability — the variation in time between heartbeats. Measured nocturnally during stable sleep stages.
HRV (Heart Rate Variability) is the variation in time between consecutive heartbeats, measured in milliseconds. Garmin measures it nocturnally during stable sleep stages, when autonomic state is least disturbed by movement, breathing variation, or cognitive load. HRV is the most sensitive single biomarker of nervous system state. Higher HRV signals parasympathetic dominance — recovery, calm, low load. Lower HRV signals sympathetic dominance — stress, training load, illness, accumulated fatigue. Personal baseline is what matters; population norms are noisy. Sleepgenic reads HRV in three ways: as a nightly value, as a trajectory across days following a training stimulus, and as a longitudinal trend against the personal baseline. The TrailGenic recovery convention — Pre / Post / Day-2 — applies directly to HRV interpretation in Sleepgenic's analytical framework.
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